Data storage media are well known. Particularly, optical data storage media in the form of compact disks are well known. Compact disks are an alternative to long-playing records and magnetic tape cassettes. The disks with which consumers are familiar are optical read-only disks and the common disk player is designed specifically for this type of disk. These disks have a reflective surface containing pits which represent data in binary form. A description of these pits and how they function is provided by Watkinson, "The Art of Digital Audio," Focal Press, Chapter 13.
Compact disks are currently produced by a pressing process similar to the process used to produce conventional long playing records. The process is referred to herein as the "mastering" process. The mastering process starts by first polishing a plain glass optical disk. The disk has an outside diameter from 200 to 240 mm, a thickness of 6 mm and undergoes various cleaning and washing steps. The disk is then coated with a thin chrome film or coupling agent, a step taken to produce adhesion between the glass disk and a layer of photo-resist, which is a photo-sensitive material. Data on a compact disk master tape are then transferred to the glass disk by a laser beam cutting method.
The glass disk is still completely flat after it is written on by the laser beam because pits are not formed until the glass is photographically developed. The disk surface is first made electrically conductive and then subjected to a nickel evaporation process. The disk, typically known as the glass master, then undergoes nickel electrocasting, a process which is similar to that used in making analog phonograph records. A series of metal replications follow, resulting in a disk called a stamper. The stamper is equivalent to a photographic negative in the sense that it is a reverse of the final compact disk; that is, there are now bumps where there were pits. This stamper is then used to make a pressing on a transparent polymer such as polyvinyl chloride, poly(ethyl-metacrylate) or a polycarbonate. The stamped surface is then plated with a reflective film such as aluminum or another metal, and finally a plastic coating is applied over the film to form a rigid structure.
The player operates by focusing a laser beam on the reflective metal through the substrate and then detecting reflected light. The optical properties of the substrate, such as its thickness and index of refraction, are thus critical to the player's detection systems and standard players are designed specifically with these parameters in mind.
The pits increase the optical path of the laser beam by an amount equivalent to a half wavelength, thereby producing destructive interference when combined with other (non-shifted) reflected beams. The presence of data thus takes the form of a drop in intensity of the reflected light. The detection system on a standard player is thus designed to require greater than 70% reflection when no destructive interference occurs and a modulation amplitude greater than 30% when data is present. These intensity limits, combined with the focusing parameters, set the criteria for the compact disks and other optical data storage media which can be read or played on such players. Media on which data can be recorded directly on and read directly from have a different configuration and operate under a somewhat different principle. One example is described in U.S. Pat. No. 4,719,615 (Feyrer et al.).
As optical information recording media of this type, compact disks (herein referred to simply as "CD") have been practically developed and widely used as optical information recording media of ROM (read only memory) type wherein pits are already formed on a light transmitting substrate by means of, for example, a press and a reflective layer of a metal is formed on the surface having such pits. As a further progress from such a ROM type, optical information recording media have been proposed on which information can be recorded by irradiating a laser beam to the substrate as the user requires. For Example, Japanese Patent Publication No. 89605/1979 discloses an optical information recording medium which comprises at least a transparent substrate, a light absorptive layer containing a coloring matter formed on the substrate and a light reflective layer formed on the light absorptive layer, and on which information can optically be recorded and from which the recorded information can be reproduced.
To conduct the reproduction by commercially available CD players, optical recording media must be able to produce read-out signals which satisfy the CD standards which are accepted world wide. To satisfy the CD standards, typical requirements are that the reflectance is at least 70%; the block error rate is at most 3.0.times.10.sup.-2 ; and when a push-pull method is employed for tracking pits, the push-pull valve is from 0.04 to 0.07. However, none of the conventional recording media comprising a substrate having a pregroove, a light absorptive layer containing a coloring matter formed on the substrate and a light reflective layer formed on this absorptive layer, uses all the aspects of the CD format satisfying the various conditions prescribed by the CD standards.
Recently a new standard has emerged as prevalent, the digital video disk (DVD). The specifications for the digital video disk (DVD) are as illustrated in Table 1, as well as a comparison with compact disk (CD) specifications.
TABLE 1 ______________________________________ CD DVD ______________________________________ Disc diameter 120 mm 120 mm Disc thickness 1.2 mm 1.2 mm Disc structure Single substrate Two bonded 0.6 mm substrates Laser wavelength 780 nm (infrared) 650 and 635 nm (red) Numerical aperture 0.45 0.60 Track pitch 1.6 um 0.74 um Shortest pit/land 0.83 um 0.4 um length Reference speed 1.2 m/sec. CLV 4.0 m/sec. CLV Data layers 1 1 or 2 Data capacity Aprox. 680 megabytes Single layer: 4.7 gigabytes Dual layer: 8.5 gigabytes Reference user data Mode 1: 153.6 1,108 kilobytes/sec, rate kilobytes/sec nominal Mode 2: 176.4 kilobyters/sec Video data rate 1.44 megabits/sec 1 to 10 megabits/sec (video, audio) variable (video, audio, subtitles) Video compression MPEG1 MPEG2 Sound tracks 2 Channel-MPEG Mandatory (NTSC) 2-channel linear PCM 2-channel/5.1- channel AC-3. Optional: up to 8 streams of data available Subtitles Open caption only Up to 32 languages ______________________________________
Sony and Philips launched the Compact Disc (CD) digital audio format over a decade ago and gave the world its first taste of digital entertainment. CD has enjoyed great success and among music companies as well as hardware manufacturers. To date, over 120 million CD players and 3 billion CD's have been sold in the United States alone. From its origins as a music format, Compact Disc has grown to encompass computer applications (CD-ROM), imaging applications (Photo CD), and video game applications.
In September 1995, nine companies joined force to create a single, unified standard for the emerging DVD format. The format enjoys the enthusiastic support of the world's major electronics companies. As a music format, DVD meets the specific and detailed requests of the music companies as well as hardware manufacturers. As a movie playback format, DVD meets the specific and detailed requests of the motion picture industry's Studio Advisory Committee. As a computer ROM format, DVD meets the specifications of the computer industry's Technical Working Group. No other product has enjoyed such broad international support across so many industries prior to launch.
Many accolades have been touted for the DVD format. Like CD, DVD is 120 mm (43/4 inches) in diameter and 1.2 mm thick. The new DVD players will be able to play the billions of existing music Cds. DVD software can be replicated using existing CD production facilities. Using the existing form factor reduces the re-tooling required to manufacture DVD Players and DVD-ROM drives. Non-contact laser optics mean playback without wear and tear. A disc-based format means the kind of split-second random access that no tape format can match. As with Compact Disc, DVD will be durable, and tolerant of dust, dirt and fingerprints. The DVD standard defines a disc that maintains the overall dimensions, look and feel of the current Compact Disc. Some of these similarities will be unmistakable to consumers experiencing DVD for the first time. Others are less apparent, but equally important to the expected rapid and successful introduction of DVD.
DVD holds seven times the data of a CD: 4.7 gigabytes per layer, as compared to 680 megabytes for CD. DVD offers a dual-layer, single-side option, for even higher capacity: 8.5 gigabytes on a single side. Every DVD is a bonded disc, composed of two 0.6 mm substrates joined together. Compared to CD, DVD uses smaller pits and a more closely spaced track. The result is a significant increase in data density. The higher Numerical Aperture (NA) lens of DVD helps the laser focus on the smaller pits. Almost every aspect of DVD was developed and refined to achieve the seven-fold increase in data capacity and data density. Refinements include smaller pit dimensions, a more closely-spaced track (finer "track pitch"), and a shorter-wavelength laser. Conventional CD players and CD-ROM drives use a laser that emits invisible, infrared light at the wavelength of 780 nanometers. The new DVD players and DVD-ROM drives use a laser that emits red light at 650 and 635 nm. The shorter wavelengths are better suited to reading the smaller, more densely packed pits. The laser assembly has also been refined with a higher Numerical Aperture (NA) lens, resulting in a narrower, more tightly focused laser beam. Even more significantly, DVD's digital modulation and error correction schemes have been specifically designed to support the increase in capacity. The 8-to-16 (EFM PLUS) modulation scheme is highly efficient and ensures backward compatibility with current discs as well as with future rewritable media. And the RS-PC (Reed Solomon Product Code) error correction system is approximately 10 times more robust than the current CD system.
The DVD format offers both single-sided, single layer discs and single-sided, dual-layer discs. On a single-sided, dual-layer disc, the laser first shines through the nearer, semi-transmissive layer to track the deeper layer of pits. The laser then switches focus to read the semi-transmissive layer. For future applications that may demand even higher on-line capacity, the DVD specifications call for a single-sided, dual-layer disc option, which nearly doubles the standard disc's capacity to 8.4 gigabytes; more than 12 times the data of a standard Compact Disc. This single-sided, dual-layer disc incorporates a semi-transmissive film which coats the layer of pits that is closest to the laser pickup. Therefore, when the laser "plays" the deeper data layer, it is actually reading through this semi-transmissive material. At the end of the first (deeper) layer, the laser pickup instantly changes its focus and begins reading the second (semi-tranmissive) layer. An electronic buffer ensures that there is seamless playback between layers.
Just as the original CD created a revolution in audio, DVD will raise the standard for home video picture quality. DVD delivers the best color, sharpness and clarity in home video, surpassing the Laserdisc standard. DVD also offers high resolution, with exception rendering of fine picture detail. Video distortion is extremely low, which reduces unwanted color "noise." Because the recording format is component video, as opposed to NTSC composite video, the pictures are free of the well-known drawbacks of NYSC--artifacts including dot crawl and cross color distortion. And because DVD is an optical format, the picture quality doesn't degrade over time and repeated use. The CCIR-601 digital video standard specifies a video rate of 167 megabits per second. At this rate, the 4.7 gigabyte capacity of a standard DVD could only store roughly 4 minutes of digital video. Thus, some form of data compression is required. DVD takes advantage of a sophisticated compression technology called MPEG2. It's a set of flexible compression standards, the second to emerge from the Moving Picture Experts Group (MPEG). MPEG2 works by analyzing the video picture for repetition, called redundancy. Over 95% of the digital data that represents a video signal is redundant, and can be compressed without visibly harming the picture quality. By eliminating redundancy, MPEG2 achieves superb pictures at far lower bit rates. As implemented for DVD, MPEG2 encoding is a two-stage process, where the signal is first evaluated for complexity. Then, higher bit rates are assigned to complex pictures and lower bit rates to simple pictures, using an "adaptive," variable bit-rate process. The DVD format uses 4:2:0 component digital video compressed to bit rates with a range of up to 10 megabits per second. Although the "average" bit rate for digital video is often quoted as 3.5 megabits per second, the actual figure will vary according to movie length, picture complexity and the number of audio channels required. A single-layer, single-sided DVD has enough capacity to hold two hours and 13 minutes of spectacular video on a 43/4-inch disc. At the nominal average data rate of 3.5 megabits per second, this still leaves enough capacity for discrete 5.1-channel digital sound in three languages, plus subtitles in four additional languages. Including video, audio and subtitles, the total average data rate is estimated to be 4.962 megabits per second. And because it's single-sided, DVD can store all this with no need to flip the disc over.
DVDs released in the United States will be capable of carrying Dolby.RTM. AC-3.TM. digital audio sound tracks with either 2 or 5.1 channels. Unlike Dolby Pro Logic.RTM. coding, Dolby AC-3 multi-channel sound provides five completely separate (discrete) channels: Left, Center, Right, Left-Rear and Right-Rear, plus a common Subwoofer channel. Dolby AC-3, which uses a digital bit rate of 384 kilobits per second, is already well accepted among videophiles and home theater enthusiasts. As a true digital system, DVD offers high quality sound, with outstanding dynamic range, vanishingly low distortion, wide frequency response and wow and flutter beneath the threshold of measurement. As an option to Dolby AC-3 sound, DVD also enables producers to choose 16-bit linear, CD quality stereo sound with Dolby Pro Logic encoding. And to facilitate international distribution of movie discs, DVD makes possible up to eight languages and 32 sets of subtitles.
The design of DVD reflects the widespread success of CD-ROM as a data carrier for computer programs, databases, multimedia software and video games. As computer processing power continues to increase, the need for higher capacity media is becoming increasingly apparent. And expanding markets for high-resolution graphics and full motion video continue to push the limits of existing storage media.
The DVD-ROM format has specific advantages. First is greater capacity. With 4.7 gigabytes of storage capacity on a single layer and 8.5 gigabytes on a dual-layer disc, DVD-ROM offers more than 12 times the capacity of CD-ROM. Also, backward compatibility is a great advantage. New DVD-ROM drives will play the tens of millions of existing CD-ROMs. On-line availability creates an advantage. DVD-ROM delivers its increased capacity on a single side; consumers will not need to turn the disc over and computer manufactures will not need to re-design their products to fit new drives. Faster data transfer will be available. Even a standard DVD-ROM drive blasts along at higher data transfer speeds than even the fastest current CD-ROM drive. Future recordability is an enhancement. DVD-ROM will be compatible with future high density recordable (DVD-Write Once) and rewritable (DVD-Rewritable) media. Affordable cost is an advantage for consumers and manufactures. DVD-ROM will ultimately be priced comparably with current CD-ROM products.
It is, therefore, a feature of the present invention to provide a method of recording data for concealing the data for later playback upon demand on a DVD.
Additional features and advantages of the invention will be set forth in part in the description which follows, and in part will become apparent from the description, or may be learned by practice of the invention. The features and advantages of the invention may be realized by means of the combinations and steps particularly pointed out in the appended claims.